A motor vehicle conventionally has an internal combustion engine with at least one injector. In particular, wear phenomena or deposits cause injection parameters, such as, for example, the actual opening period or the actual degree of opening of the injector, to change. As a result, the actual injection quantity also changes during the service life of the injector. In order to maintain the strict emission standards and to be able to continue to drive with a high fuel efficiency, the injection system of the internal combustion engine must therefore be capable of injecting a defined fuel quantity precisely over the service life of the injector. This means stringent requirements made of the injection stability and injection accuracy of the injector.
However, since the properties of the injector unavoidably change over its service life, online adaptation of the injection control parameters is necessary.
A known approach for adapting the injection parameters detects a speed signal of the crankshaft and/or of the internal combustion engine. This is due to the fact that if combustion occurs in the internal combustion engine, the engine crankshaft is accelerated. The change in the acceleration is determined by means of the corresponding speed signal, and the injection parameter is corrected.
These known solutions require an operating state of the internal combustion engine in which the fuel supply is switched off, i.e. no normal injection takes place. During this operating state a test injection pulse is implemented and the acceleration is used as an indicator for the injected fuel quantity.
A new approach is described in German application 10 2010 014 320. In said document, the test injection pulse is performed during a normal operating state, preferably during an idling state or an uncoupled state of the internal combustion engine. While the internal combustion engine is in a steady idling state, the rotational speed control (DR) is frozen for at least one injection cycle. “Freezing” means here that the injection parameters for all the normal injection pulses are the same as the injection parameters of the last or preceding injection cycle, apart from the defined test injection pulse. In this way, calibration variants for various transmission types are avoided.
FIGS. 1A and 1B show an injection pattern of such a method plotted against the time t. From FIGS. 1A and 1B it is clear that in the illustrated example an injection cycle comprises four segments (seg0 to seg3). The internal combustion engine is therefore a four-cylinder internal combustion engine.
FIG. 1A shows the normal injection cycle without a test injection pulse during the idling phase. The rotational speed control (DR) is activated. The lack of a test injection pulse is characterized by the reference symbol 1.
FIG. 1B shows the test injection cycle which is a precise copy of the injection configuration of the normal or preceding injection cycle (for example the injection time, the injection position etc.). This means that the rotational speed control (DR) is deactivated for the test injection cycle, that is to say is frozen. The additional test injection pulse in the segment 0 (seg0) is characterized by the reference symbol 3.
The combustion brought about by the test injection pulse is calculated from the difference between the acceleration signal of the first four segments (seg0 to seg3 from FIG. 1A) and the acceleration signal of the following four segments (seg0 to seg3 from FIG. 1B).
FIG. 4A shows an illustration of the combustion signal determined according to the prior art. The injection time Ti is plotted in milliseconds on the x axis, and the combustion signal CMB_STC is plotted on the y axis. The fuel pressure in the injection system is 40 MPa. The combustion effect of a test injection pulse has been determined for five different test injection pulse lengths, wherein the fault bars respectively indicate the standard deviation at the measuring point.
Freezing is performed during the test injection cycle in order to exclude any influence of injection pulses other than the test injection pulse in the test injection cycle on the resulting acceleration signal. Otherwise, the rotational speed control (DR) would, as a reaction to the test injection pulse, falsify the segment times of the segments following the test injection pulse. As a result, the exclusion of this negative influence is performed by selecting precisely the same injection parameters as in the preceding injection cycle. In this case it is ensured that the determined difference between the acceleration signals has been caused solely by the test injection pulse.
However, it is disadvantageous that the method described in this document requires switching off (deactivation or freezing) of the rotational speed control (DR). This increases the complexity of the application software owing to the large number of interactions between the functional assemblies of the speed controller of the internal combustion engine (ENSC) and the implementation of the injection (INJR).